Please suggest a best country or island to visit in December......

Uma, no offense, but that is a very broad question. 'Best' in what way? It would be very helpful to know what you like to do on vacation, how cold tolerant you are, how far are you willing to travel, how many days, late or early December? For many countries, Christmas to New Year (Jan 1) is a busy time and high season for prices. Often even further price hikes happen for Dec 31-Jan 1.

The world is totally available to you, but we don't know anything about you in order to make any recommendations that might be actually relevant to you. It would be just as relevant to say 'put a pin into a world map', but that isn't very helpful.
Please expand on your expectations or desires.

Best island to visit in December? Manhattan.

Hi Maria, Am sorry for not being specifically. Am planning for my vacation 29th December to 11th January 2019. Last years i have spent my vacations in Europe. So this time preferred not very cold weather. But if its something worth to see like northern lights I do not mind the cold weather. I know the prices are high in this season but unfortunately this is the scheduled vacation since this is the one out of three which i can get leave from my work. So despite the cost please suggest me few places to visit. If its some where like Palawan, Philippines i would be really happy. Photography is my hobby, love to enjoy nature stuff and love to enjoy night life also.
When it comes to Asia i already visit Thailand, Vietnam, Maldives and Philippines. So please suggest without these places also.

Thank you Maria :)

No offense intended, but this is an impossible question to answer (and it makes me wonder if it's sincere or trolling or something else).

"Please suggest the best food in the world for my next dinner. I am planning to eat dinner tonight around 7:00 pm. Last night I had lasagna, so this time I prefer something without so many carbs. But if it's very special, I don't mind more pasta and noodles. Please give me some suggestions because I will be hungry soon."

See any parallel?

Thanks for the extra information. Still so many choices, but let's see. I had to google Palawan, as I have never been to your side of the world, but it is lovely. Personally, if that was on my door step, I wouldn't look further, but that is irrelevant.
Perhaps somewhere in the Caribbean you can find the beaches and warmth, but they will have more crowds and infrastructure. The cities may be smaller on some islands. Panama can offer a variety of environments and Costa Rica has some cities as well as beautiful nature and birds to photograph.
Yukon or North West Territories in Canada offer short Northern lights visits to lodges. You can even rent cold weather clothing. It won't be as cold as February, but I can't see spending 2 weeks there. You could split it with somewhere else. Canada, however, is cool to down right frigid in January, so maybe Hawaii. Say, that would be a good option.

Brazil!! You can get to Rio with only one change of planes from Colombo! The food is delicious, the beaches are warm at that time of year, and the people are beautiful. Have a great trip.

Kauai or Maui

Costa Rica. Once you're outside San Jose, it's a nature lover's paradise.

All of these suggestions look like they are a long way from Sri Lanka, but what about New Zealand if you want to see cities or Bali if you want a beach.

Australia is amazing

Argentina, Chile, Uruguay. The airfare will be expensive, but once you are there, the hotels will be cheap and restaurants prices are good compared to US or Europe. Remember to bring all your supplies with you, as store prices are about double US prices in Argentina and Uruguay (22% VAT).

You don't have to stray too far from home to find cool weather, beautiful landscapes to photograph, and pleasant walks thru the forest. There are non-stop flights from Colombo to both Bangkok and Kuala Lumpur, and from either it would be a relatively short connection to get to the highlands of either country. The Cameron Highlands of Malaysia are as yet an undiscovered little gem of a destination. The Brits established huge tea plantations in the area which continue to the present day. There are some terrific walks and hikes thru the hills that will keep you occupied for weeks.

For the Northern Lights I'd suggest Alaska or Iceland, you can get package air and hotel deals from major US cities and probably elsewhere. I guess Norway would be a good choice too.

But it's a long way to go with the risk that it will be cloudy and you'll just be cold. Instead, consider Hawaii or Australia and/or New Zealand. Or how about a cruise between Buenos Aires and Santiago?

Uma, you are from Sri Lanka. I am sure there are plenty of nice beaches there although I haven't been there yet. If it is that what you want I am sure it would be much cheaper and more convenient than to fly for many many hours somewhere else for something what you already have home. Personally I am thinking about Tahiti or maybe even better Fiji. Both are a long flight from Seattle but I don't have anything like that here and you do. But if you want an island which is totally different than yours I suggest Iceland. If you are lucky you would even see Northern lights.

Thanks All....... :)